1. Field of the Invention
This invention relates to static electricity dissipation drains for dissipating static charges within structures such as storage tanks to minimize a build-up of static electrical potential within the structure that might create an electrical spark within the storage tank.
2. Description of the Background Art
As set forth in my prior patent, U.S. Pat. No. 4,910,636, the disclosure of which is hereby incorporated by reference herein, static electricity dissipators have been used to dissipate electrical charges during a thunderstorm to thereby minimize the likelihood of a lightning strike that might otherwise occur due to the electrical potential between the earth and the atmosphere. My prior static electricity dissipator comprised an electrically-conductive base member having a plurality of fine conductive wires emanating therefrom in a uniform, mushroom-shaped configuration. The tubular base member was typically affixed to roof locations above the structure to be protected such as at locations in which conventional lightning rods would be installed. My static electricity dissipator has achieved substantial commercial success, and has been widely accepted throughout the industry.
U.S. Pat. No. 4,605,814, the disclosure of which is hereby incorporated by reference herein, discloses a lightning deterrent which comprises a cable having a multiplicity of fine conductive wires captured within the strands of the cable to emanate therefrom in a brush-like manner. During use, the cable is formed in a circular or other configuration and mounted about the periphery of the structure to be protected. The terminal ends of the multiplicity of fine conductive wires function to dissipate electrons to the atmosphere, thereby minimizing the electrical potential differential between the structure and the atmosphere. The likelihood of a lightning strike is thereby minimized.
Similar dissipators in which fine conductive wires are captured within the strands of a main cable are disclosed in the following U.S. Patents, the disclosures of each of which are hereby incorporated by reference here.
Pat. No.TITLEU.S. 1,757,172AerialU.S. 2,631,189Static Wick DischargerU.S. 3,617,805Low-Noise Static Discharger DeviceU.S. 4,180,698System and Equipment for Atmospherics ConditioningU.S. 4,605,814Lightning DeterrentU.S. 5,638,248Static DissipatorU.S. 6,307,149Non-Contaminating Lightning Protection SystemU.S. 6,369,317Safer Lightning Rod and Warning SystemU.S. 6,943,285Bipolar Discharge-Dissipation Lightning Air TerminalsU.S. D478,294Lightning Dissipation AssemblyU.S. D478,295High Dissipation Discharge TerminalU.S. D478,525Point Discharge Dissipation Terminal
In addition to dissipating static electricity for lightning protection for buildings, it is likewise known that storage tanks, which store a combustible fluid, are in need of lightning protection. Representative patents disclosing lightning dissipators for protection of storage tanks and other structures are disclosed in the following U.S. Patents, the disclosures of each of which are hereby incorporated by reference herein.
Pat. No.TITLE1,617,788Device for Preventing Electrical Ignition of Stored InflammableFluids1,743,788Apparatus for Treating Flotant Material1,974,315Lightning Protection for Storage Tanks5,694,286Lightning Protection Device6,815,606Bipolar Multi Electrostatic Inducing Discharge-DissipationLightning Air Terminals
More specifically, during filling of a storage tank, particularly one composed of fiberglass or metal lined with insulative dielectric material, it is known that a static charge is created between the fluid inflow droplets and the inner wall of the storage tank. It is also known that as the static electrical potential accumulates, an electrical spark could eventually be created within the storage tank, and could thereby cause the flammable liquid therein to ignite or explode.
Unfortunately, apart from lightning protection, there has been an unsatisfied need for a static electricity dissipator for reducing static electrical potential within the storage tank or other structure itself, particularly when the tank or structure is manufactured of a non-conductive material such as fiberglass (e.g., a fiberglass saltwater disposal (SWD) tanks) or metal lined with an electrically insulative material. Known prior art systems that employ a Carbon Veil, Chains or a Conductive Paint or that consist of a Catenary System or an Early Streamer Emitting System are discussed as follows.
Carbon Veil is a conductive strip woven into a fiberglass tank with a grounding lug provided near the base of the tank. The intent is to dissipate static charge from the stored product onto the strip. The drawback of this system is that it presents a flat surface to, and is not in direct contact with, the stored product. Charge more readily dissipates into a liquid off small radius electrodes than off flat surfaces, limiting the effectiveness of the veil. If adjacent wraps of the veil do not overlap, it presents the possibility of arcing between wraps during a lightning strike or ground fault. The carbon veil does not provide bonding to miscellaneous masses of inductance on the tank. Neither does it provide air terminals (lightning rods) or a full-size conductor to ground.
Chains (or other appliance suspended in tank) are intended to dissipate static charge from the stored product onto the chain or other appliance. The drawback of this system is that it presents a flat (curved) surface to the stored product. Charge more readily dissipates into a liquid off small radius electrodes than off flat surfaces, limiting the effectiveness of the appliance. The chain or other appliance does not provide bonding to miscellaneous masses of inductance on the tank. Neither does it provide air terminals (lightning rods) or a full-size conductor to ground.
Conductive Paints are employed but only to coat the outside of the tank. Therefore, it cannot dissipate static charge from the stored product. Conductive paint may help by providing a path for energy from a direct lightning strike down the tank exterior. However, this division of current over the face of the painted surface is compromised, as there is only one or two ground lugs providing a path to ground at the base of the tank. Additionally, the painted surface will be only marginally effective in serving as a lightning attachment point. If lightning attaches to the tank, the paint will probably not be thick enough to prevent melt-through of the fiberglass, as it does not meet lightning protection code requirements (NFPA 780-3.6.1.3).
A Catenary System consists of grounded masts or poles supporting a wire or wires over the site. This type of system is primarily intended to protect electric power utility company transmission and distribution lines by intercepting what would otherwise be direct strikes to the phase conductors. The overhead wires have no effect on streamer formation from the tanks, and therefore do not affect the likelihood of a direct strike to the tanks They are merely intended to “get in the way” of a direct strike, intercepting and conveying it to ground. When used to protect tanks or other structures, this system cannot mitigate secondary effect arcing, the primary cause of ignition. In fact, if a catenary system performs exactly as designed and intercepts a direct strike, it maximizes the likelihood of secondary effect arcing across the tank and appurtenances by bringing the lightning energy to ground near the base of the tank. The catenary system also has no effect on the static charge on the stored product, does not provide bonding to miscellaneous masses of inductance on the tank, and does not provide purpose-designed air terminals on the tank or tank battery.
An Early Streamer Emitting System uses a small number of air terminals, usually a single air terminal, to protect an extended area. This type of air terminal works by emitting a streamer early in the streamer formation phase of a lightning strike. The streamer will therefore reach the downward reaching stepped leaders before any other, thereby becoming the preferred lightning attachment point. They often are labeled with names inferring that they protect the area by keeping away direct lightning strikes. Actually, the opposite is true. They attract lightning to themselves and to the site. Therefore, lightning will tend to attach to the ESE air terminal rather than to the tanks and other structures. However, lightning attachment is not the primary cause of ignition at the sites. Secondary effect arcing is the primary cause of ignition. As these devices attract lightning to themselves, they actually cause maximum secondary effect current flow right at the site, introducing, not preventing, the primary cause of ignition.
Lightning protection for external floating roof tanks has been the subject of much discussion in recent years. The American Petroleum Institute has recently devoted much time and study to this subject and has promulgated API 545—Lightning Protection for Hydrocarbon Storage Tanks
By way of background, a lightning strike consists of two components: a short duration, high-energy spike which is then followed by a longer duration, lower energy tail. While the high-energy spike is impressive, it is the lower energy, long duration component that is actually responsible for ignitions in external floating roof tanks
More specifically, the roof of the tank floats on pontoons on the stored product. It is centered in the tank shell by centering shoes. Vapor is contained by a primary and a secondary seal. These tanks have traditionally been equipped with flexible, stainless steel grounding shunts spaced at frequent intervals around the perimeter of the floating roof. Additionally, the floating roof is usually bonded to the tank shell with one grounding conductor run along the stairway from the top of the tank shell to the floating roof
Lightning becomes an issue when it strikes either the floating roof, the tank shell, or nearby. Ignition is not normally caused by the heat of the lightning channel igniting venting vapors. It is caused by arcing from the secondary effect of lightning. A thunderstorm is an electrically charged cloud mass, with a charge, usually negative, at its base. That charge induces an opposite charge, usually positive, on the surface of the earth beneath it. When lightning attaches to a tank or other object on the surface of the earth, the charge at the point of attachment changes dramatically and almost instantly. The surrounding ground charge rushes toward the point of the strike. If that in-rush of charge crosses a gap, it may arc. If that gap is between the floating roof and the side of the tank shell, and there are flammable vapors present, those vapors may ignite.
Another way of looking at this phenomena is to consider a lightning attachment to the shell of the tank. The tank shell changes potential almost instantly. The floating roof, being somewhat electrically isolated from the shell, does not. That difference in potential between the floating roof and the tank shell must equalize. Unless a preferred path is provided, a potential equalizing arc may occur, once again igniting any flammable vapors present.
Presently, most external floating roof tanks are equipped with flexible stainless steel grounding shunts around the perimeter of the floating roof. These shunts are attached to the roof, and bent upward and outward to press against the tank shell wall. They ride against the tank shell wall, up and down as the roof rises and falls. The electrical contact to the wall is adequate only when the tank is new and the wall is clean. After a few trips up and down, the tank wall becomes coated with a variety of substances that compromise the electrical bond. Because of the short length and frequent spacing of these shunts, they are the preferred path of equalization between the floating roof and tank shell for the high-energy short duration component of the lightning strike. API 545 recommends employing these shunts for this purpose. However, because of the contaminants on the tank wall, these shunts tend to emit a shower of sparks when they perform their intended function. One solution suggested by 545 is to relocate these shunts so they are submerged under the stored product and there is no oxygen available at the source of the sparks to support ignition. However, submerging the shunts creates other problems when the roof is landed.
Further, to address the lower energy, long duration component of the lightning strike, API 545 recommends the installation of by-pass conductors between the floating roof and tank shell at intervals not to exceed 100′ around the roof perimeter. These conductors provide a low-resistance bonding path between the roof and tank shell, and are intended to prevent ignition-causing arcs generated by this current flow.
In summary, by-pass conductors address the lower-energy longer duration component of the lightning discharge and simply attaching a length of conductor from the edge of the floating roof to the top of the tank shell is adequate. Unfortunately, however, the by-pass bonding conductors must be kept out of the way as the floating roof rises and falls. One embodiment comprised a grounding reel similar to that used to bond a fuel truck to an airplane. This grounding reel employed a flat, braided, tinned copper strap. The strap offered lower surge impedance than a round conductor, and, as the strap retracted into the reel, it was pressed against the inner windings of strap, effectively shortening the overall length of the conductor. Unfortunately, grounding reels were of questionable durability and were costly.
Accordingly, there presently exists a need for a static electricity dissipator drain for use inside a structure such as a storage tank to dissipate the static electrical potential that may accumulate therein and otherwise create an electrical spark in the structure.
Therefore, it is an object of this invention to provide an improvement which overcomes the aforementioned inadequacies of the prior art devices and provides an improvement which is a significant contribution to the advancement of the static electricity dissipator art.
Another object of this invention is to provide a static electricity dissipation drain for storage tanks having a fixed roof or a floating roof.
Yet another object of this invention is to provide a static electricity dissipation drain for storage tanks composed of metal, fiberglass, plastic or lined metal.
Another object of this invention is to provide a static electricity dissipation drain for storage tanks to bond the stored product and suspended droplets in the vapor space to the bonded mass of the tank.
Another object of this invention is to provide a static electricity dissipation drain for storage tanks to dissipate the static charge in the stored product and suspended droplets in the vapor space, preventing it from building to an incendive level.
Another object of this invention is to provide by-pass conductors from the edge of the floating roof to the top of the tank shell to address the lower-energy longer duration component of the lightning discharge that is kept out of the way as the floating roof rises and falls.
The foregoing has outlined some of the pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.